In this Jan. 7, 1997 photo, House Speaker Newt Gingrich of Georgia and his wife Marianne leave their home for Capitol Hill.

By msnbc.com staff and wire reports

Updated Thursday at 1:15 p.m. ET

In excerpts of an interview scheduled to air on Thursday night, Marianne Gingrich claims that her ex-husband, Newt Gingrich, sought an "open marriage" before their own relationship ended.

In the interview with ABC News, Marianne Gingrich claims that when the then-House speaker told her of a long-term, six-year affair he was having with a congressional staffer (Callista Bisek, now his third wife), he asked if he could remain married to Marianne and continue the affair.

In an exclusive interview with ABC News, Newt Gingrich's ex-wife Marianne Gingrich claimed the former speaker's asked her for an "open marriage" and to allow him to continue his affair with then-mistress Calista.

Marianne Gingrich, the former speaker's second wife, also alleged that her ex-husband conducted his affair "in my bedroom in our apartment in Washington."

She also said Gingrich moved to divorce her just months after she was diagnosed with multiple sclerosis. "He also was advised by the doctor when I was sitting there that I was not to be under stress," she said. "He knew."

According to Marianne Gingrich, her relationship with him began while Newt Gingrich was still married, but in divorce proceedings, with his first wife, Jackie. At the time, Jackie Gingrich was being treated for cancer.

She asked in that conversation with The Washington Post, "How could he ask me for a divorce on Monday and within 48 hours give a speech on family values and talk about how people treat people?"

In an interview with NBC's TODAY on Thursday, before the ABC News excerpts were released, Newt Gingrich vowed that he won't say "anything negative" about his former wife.

Former House Speaker Newt Gingrich talks to TODAY's Ann Curry about his presidential candidacy, defends his comments about the poor and says he's not going to criticize his ex-wife, who has promised to give an explosive interview to ABC News.

According to The Washington Post, at a campaign stop in South Carolina on Thursday, he called the statements by Marianne Gingrich, "tawdry and inappropriate," but wouldn't go further. His third wife, Callista Gingrich, stood a few feet behind him.

As word spread Wednesday about the interview with Marianne Gingrich, the campaign responded with something of a "prebuttal."

In advance of that interview, the campaign released a statement Wednesday night from Gingrich's daughters from his first marriage, Kathy Lubbers and Jackie Cushman.

They wrote:

The failure of a marriage is a terrible and emotional experience for everyone involved. Anyone who has had that experience understands it is a personal tragedy filled with regrets, and sometimes differing memories of events.

We will not say anything negative about our father’s ex-wife. He has said before, privately and publicly, that he regrets any pain he may have caused in the past to people he loves.

ABC News or other campaigns may want to talk about the past, just days before an important primary election. But Newt is going to talk to the people of South Carolina about the future– about job creation, lower taxes, and about who can defeat Barack Obama by providing the sharpest contrast to his damaging, extreme liberalism. We are confident this is the conversation the people of South Carolina are interested in having.

Our father is running for President because of his grandchildren – so they can inherit the America he loves. To do that, President Obama must be defeated. And as the only candidate in the race, including Obama, who has actually helped balance the national budget, create jobs, reform welfare, and cut taxes and spending, Newt felt compelled to run - to serve his country and safeguard his grandchildren's future.